Cool roofs in cool climates
Cool roofs in cool climates
No matter where cool roofs are installed, they cut down on the urban heat island effect, however they do not always lower a building’s carbon footprint. In climates where there are more heating days than cooling days, white reflective roofs are not typically a worthwhile investment in terms of energy efficiency or savings. The cooling benefits of a highly reflective roof surface do not outweigh the winter month heating benefits of a less reflective, or black, roof surface in cooler climates. Heating accounts for 29% of commercial buildings' yearly energy consumption, while air conditioning only accounts for 6% of that same yearly energy consumption. Therefore, in cooler climates, it is more beneficial to utilize a dark-colored roof surface to help lower heating costs, which far outweigh annual air conditioning expenses. Energy calculators generally show a yearly net savings for dark-colored roof systems in cool climates. Often times, reflective roofing materials get dirty, and their reflective benefits diminish, after only a few short years. Without a proper maintenance program to keep the material clean, reflective roofing materials seldom provide the energy-saving benefits that could be fully experienced based on their initial SRI.
Additionally, higher R values for insulating materials can lessen the impact of roof surface color. Snow on roofs also provides insulation, but it also adds considerable weight to the roofing assembly, which may not have been accounted for in the initial design. For a medium density of snow the resistance per 25 mm is about .110 (m2-°C)/W, 300 mm of snow cover can provide an equivalent of 50 mm of good insulating material. Cool roofs contribute to the retention of snow on roofs in moderate snow fall areas. Dark-colored roofs heat up more quickly and therefore help melt rooftop snow. There can be a 26 °C differential in membrane temperature between areas having 300 mm of snow cover compared to areas having no snow.
Research and practical experience with the degradation of roofing membranes over a number of years have shown that heat from the sun is one of the most potent factors that affects durability. High temperatures and large variations; seasonally or daily, at the roofing level are detrimental to the longevity of roof membranes. Reducing the extremes of temperature change will reduce the incidence of damage to membrane systems. Covering membranes with materials that reflect ultraviolet and infrared radiation will reduce damage caused by u/v and heat degradation. White surfaces reflect more than half of the radiation that reaches them, while black surfaces absorb almost all. White or white coated roofing membranes, or white gravel cover would appear to be the best approach to control these problems where membranes must be left exposed to solar radiation.
There are some studies that have shown that reflective roofs are not always best in cool climates. Benchmark Inc. did a study in five different cities and used the energy star calculator and the DOE calculator to find the annual savings. Because the DOE calculator includes differences in heating losses, there were significant differences between the savings in all of the cities. However, in Chicago, the annual savings became slightly negative in one of the models because of heating costs. The following graph shows the results:
Calculations performed using the DOE Energy Star Calculator show that high-reflectivity, medium-emissivity roof coatings, such as aluminum roof coatings can yield greater savings in colder regions. http://www.energystar.gov/ia/partners/prod_development/revisions/downloads/roofs/RCMA-CommentLetter-081606.pdf published a blog article by Robert Reale expressing an opinion that areas where heating is more of a concern than cooling would not benefit, and so cool roofs are only appropriate in climate zones 1-3. ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers') position on reflective roofs falls in line with Mr. Reale's article. ASHRAE now promotes the use of reflective roofs only in climate zones 1-3. In zones 4 and above, darker-colored roofing materials are more beneficial. An article in ecobroker.com also does not recommend reflective roofs in cooler climates. This site is designed to aid real estate agents in finding their clients green homes.
Miller-McCune
Green roofs are another option to consider for flat roofs in cooler climates.
One issue that is rarely talked about in terms of cool/reflective roofing is "What happens to the heat/UV that is reflective from the roof surface?" Well, if it's coming from a lower building adjacent to taller buildings, the energy is likely transferred into the adjacent building. This negates the energy-saving benefits for the building with the reflective rooftop, however it increases the heat gain, and subsequent energy costs, for the adjacent building. Furthermore, studies show that heat gain through windows has more than 10x the impact on energy costs and consumption that heat gained through the roof assembly. So, the reduction in energy costs (and subsequent carbon emissions) from the building with a reflective roof is multiplied by the adjacent building that picked it up via the windows.
Types of cool roofs
Cool roofs for commercial and industrial buildings fall into one of three categories: roofs made from inherently cool roofing materials, roofs made of materials that have been coated with a solar reflective coating, or green planted roofs.
Inherently cool roofs
White vinyl roofs, which are inherently reflective, achieve some of the highest reflectance and emittance measurements of which roofing materials are capable. A roof made of thermoplastic white vinyl, for example, can reflect 80 percent or more of the sun’s rays and emit at least 70% of the solar radiation that the building absorbs. An asphalt roof only reflects between 6 and 26% of solar radiation, resulting in greater heat transfer to the building interior and greater demand for air conditioning – a strain on both operating costs and the electric power grid.
Coated roofs
One of the way to make an existing or new roof reflective is by applying a solar reflective coating on its surface. These coatings are specially engineered to reflect heat, regular white paint is not enough.
Ceramic coatings are the most well known in this domain, they provide an average reflectance of 75% to 85%. They have several limitations such as color (usually white only), which is why they are used by industrial applications mainly (warehouses, factories, ...). Their application is done by trained professionals only and may require specialized equipment and appropriate safety precautions.
High performance nanotechnology heat reflective paints are the most innovative in this field. They can reflect up to almost 95% of solar radiations, reducing a roof's heat load by an average of 30% in hot weather with as little as 200 microns in thickness (.2 mm). Working at nanotechnology levels allows thermal barrier paints like Planet Supra, for example, to offer an unlimited choice of colors in matte or glossy finish (the lighter the color, the higher the performance), easy application like any regular water-based paint and additional benefits such as self-cleaning properties thanks to Titanium Dioxide in the formulation.
Reflectivity and emissivity ratings for some reflective roof products can be found in the CRRC (Cool Roofs Rating Council) website.
Green roofs
A green roof typically consist of an insulation layer; a waterproof membrane; a drainage layer, usually made of lightweight gravel, clay, or plastic; a geotextile or filter mat that allows water to soak through but prevents erosion of fine soil particles; a growing medium; plants; and, sometimes, a wind blanket. Green roofs are classified as either intensive or extensive; some green roof designs incorporate both intensive and extensive elements.
Intensive green roofs require at least one foot of soil and appear as a traditional garden with trees, shrubs and other attractive landscapes. They are multi-layer constructions with elaborate irrigation and drainage systems. These roofs are often designed for recreational purposes and accommodate foot traffic. Intensive green roofs add considerable load to a structure and require intensive maintenance, so they are more common with large businesses or government buildings rather than free-standing homes.
Extensive roofs usually require less maintenance. The soil is shallower (less than 6 inches) and home to smaller, lighter plants such as mosses or wildflowers.
Both types of green roofs offer a variety of benefits including:
Improved air quality as the plants absorb and convert carbon dioxide to oxygen
Long lifespan - some green roofs in Europe have lasted more than 40 years
Excellent insulation
Cooled surrounding environment
A cool roof case study
In a 2001 federal study, the Lawrence Berkeley National Laboratory (LBNL) measured and calculated the reduction in peak energy demand associated with a cool roof’s surface reflectivity. LBNL found that, compared to the original black rubber roofing membrane on the Texas retail building studied, a retrofitted vinyl membrane delivered an average decrease of 24°C (43°F) in surface temperature, an 11 percent decrease in aggregate air conditioning energy consumption, and a corresponding 14 percent drop in peak hour demand. The average daily summertime temperature of the black roof surface was 75°C (168°F), but once retrofitted with a white reflective surface, it measured 52°C (125°F). Without considering any tax benefits or other utility charges, annual energy expenditures were reduced by $7,200 or $.07/sq. ft.
Instruments measured weather conditions on the roof, temperatures inside the building and throughout the roof layers, and air conditioning and total building power consumption. Measurements were taken with the original black rubber roofing membrane and then after replacement with a white vinyl roof with the same insulation and HVAC systems in place.
Programs promoting the use of cool roofs
Energy Star
ENERGY STAR is a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy designed to reduce greenhouse gas emissions and help businesses and consumers save money by making energy-efficient product choices.
For low slope roof applications, a roof product qualifying for the ENERGY STAR label under its Roof Products Program must have an initial solar reflectivity of at least .65, and weathered reflectance of at least .50, in accordance with EPA testing procedures. Warranties for reflective roof products must be equal in all material respects to warranties offered for comparable non-reflective roof products, either by a given company or relative to industry standards.
